Do you wonder where those old iMovie 2 sounds went? They're inside of the aplication package (control-click on the app and select Show Package Contents), in Contents -> Resources -> Sound effects.

Now the cool part is, you can add subfolders inside of the Sound Effects folder to place your own sounds into! I created a new subfolder titled "iMovie 2 Sounds" and then dropped in all of those great old sounds. I then added appropriately titled subfolders for my other sound file collections. I noticed no ill effects from doing this, excluding a couple of extra seconds at boot...

[Editor's note: This works exactly as described, and is quite cool! As I'm the paranoid sort, I actually copied the old sounds into the new folder instead of moving them. Sure enough, re-launch iMovie and there's the new folder with the "classic" sounds available.]

Interesting...I've got iMovie 3.0.1 setup just like I described, and in the audio panel under "iMovie Sound Effects", I get a major heading (like "Skywalker Sound Effects" and "Standard Sound Effects") for each folder in Users -> MyName -> Library -> iMovie -> Sound Effects. Each mp3 and AIFF is listed underneath its folder name.
Note that if you have iM3 open while putting the sounds in the folder, you'll have to quit and restart the application before the sounds will show in the audio panel.

Not really on topic...I would like to try this but I lost iMovie2 (along with some other older app versions) thanks to a Wintel disk crash. Anybody know where I can download the older "iLife" apps? Tried searching google but no luck.